1. Field of the Invention
The present invention relates to an asymmetric micro-porous membrane. More particularly, it relates to a micro-porous membrane having a small filtration resistance.
2. Prior Art of the Invention
Micro-porous membranes have been known for quite a long time (For example, "Synthetic Polymer Membranes" written by R. Kesting and published by McGraw Hill Co.) and widely used for filtration, etc. They are produced by using cellulose esters as starting materials as described in U.S. Pat. Nos. 1,421,341, 3,133,132 & 2,944,017, and Japanese Patent Publication Nos. 15698/68, 33313/70, 39586/73 & 40050/73, by using aliphatic polyamides as described in U.S. Pat. Nos. 2,783,894, 3,408,315, 4,340,479, 4,340,480 & 4,450,126, and German Patent No. 3,138,525, by using polyfluorocarbons as disclosed in U.S. Pat. Nos. 4,196,070, & 4,340,482, and Japanese Patent Publication (OPI) Nos. 99934/80 & 91732/83 (the term "OPI" as used herein means an "unexamined published application"), by using polysulfones as shown in Japanese Patent Publication (OPI) Nos. 154,051/81 and 86,941/81, and employing polypropylene as shown German Patent OLS 3,003,400. These micro-porous membranes are used for e.g., filtering or sterilizing cleaning water for the electronic industry, water for medicines including that used in their manufacturing processes, water for foodstuffs, etc. The uses of the membrane and their quantity consumed are increasing recently. Especialy, highly reliable micro-porous membrane are used a lot and attracting attention from the viewpoint of catching fine particles.
Such micro-porous membranes are divided into two groups, i.e., (1) the so-called symmetric membrane: the micro-pores in this membrane have diammeters which substantially do not vary in the direction of its thickness and those on both sides of its have substantially the same diameter, and (2) the so-called asymmetric membrane: its micro-pores have diameters which vary continuously or discontinuously in the direction of its thickness and the diameters of them on one side of the membrane differ from those on the other side of it.
The symmetric membrane, as explained in Japanese Patent Publication (OPI) No. 154051/80, gives a great resistance to the flow of a fluid on its filtration so that the flow rate is low (that is, only a low flow rate is obtainable per unit area, unit period of time and unit pressure differance). Also, the filtration ability of the membrane can not last long since its pores tend to be blocked easily, i.e., no anti-blocking ability.
On the other hand, the asymmetric membrane has on its surface a fine layer which contains pores of the shortest diameters, whereby fine particles of a minimum size can be substantially caught and removed here on filtration. This means that all the thickness of the membrane can be efficiently utilized as a filter so that it is possible to increase the filtration flow rate as well as to prolong the life of the membrane as a filter, when it is used with care.
In the above case, however, since the fine layer which is very important is on the surface of the membrane, it tended to get scratched or abraded, often resulting in an escape of fine particles.
To solve this problem, the fine layer having micro-pores of the shortest diameters is desired to be inside a membrane for filtration. For example, Japanese Patent Publication (OPI) No. 150402/83 proposes a structure in which two asymmetric membranes are adjusted together in layers in such a manner as their fine layer's sides come in contact. However, in such a filter of layered asymmetric membranes, when it is folded in an accordion fold and placed in a cartridge, the filtration area within the cartridge becomes small and therfore, the filtration flow rate as a module gets low. Accordingly, it has been strongly desired in this industry to invent a structure which has said fine layer inside a single membrane.
In order to overcome the above problem, the present inventors studied in detail one of the traditional methods for producing micro-porous membranes, i.e., a dry-wet method, in which a polymer solution is spread and allowed to stand in air for a certain period of time to cause a micro-phase separation whereby the diameter of the micro-pores is controlled. Such method was divided into two types, e.g., the one is a method to let the solvent used evaporate thoroughly (e.g., Japanese Patent Publication (OPI) No. 102416/80) and the other is a method in which the spread polymer solution is hardly left to let the solvent evaporate and is immersed in a solidifying bath (e.g., Japanese Patent Publication (OPI) Nos. 8887/80 & 154051/81).
Differently from these two methods, the present inventors discovered that, surprisingly enough, a layer of minimum pores can be formed at a certain depth inside a micro-porous membrane when evaporation of the solvent from the spread polymer solution is suitably controlled, while a highly accurately adjusted amount of vaper of a non-solvent to the polymer is given to said spread solution from the air so as to let the solution absorb the vaper through its spread surface, whereby a micro-phase separation is caused to occur to a certain depth below the surface of the spread solution.